(1) Field of the Invention
The present invention relates to an exhaust purifying apparatus for controlling the exhaust flow of an internal combustion engine by using an exhaust throttle valve provided in an exhaust passage of the engine, and relates to a failure prevention method for an exhaust purifying apparatus.
(2) Description of the Related Art
To reduce harmful substance emission and achieve early catalyst activation, there have been proposed technical arts, which control the exhaust flow of an internal combustion engine to promote reaction between unburned fuel components and oxygen (O2) in an exhaust system (from combustion chambers to an exhaust pipe) of the engine. For in-cylinder injection type engines, it has been found that noticeable effects can be attained by using the just-mentioned arts in combination with 2-stage combustion for performing sub-injection in addition to main injection, or a compression slight lean operation for making fuel injection in a compression stroke at a slightly lean air/fuel ratio, or secondary air supply, etc.
For a method of controlling the engine""s exhaust flow, an exhaust throttle valve is sometimes provided in the exhaust passage. With such a method, however, when attempting to control the exhaust flow in various engine operating conditions that are different in exhaust flow rate or exhaust temperature from one another, the exhaust pressure (i.e., the pressure in the exhaust passage) greatly varies if the throttle opening remains constant, resulting in an unstable reaction in the exhaust system. In addition, degraded fuel consumption, deteriorated combustion, etc. may be caused by an excessively increased exhaust pressure. Therefore, in order to attain the desired exhaust pressure, usually the exhaust throttle valve is used in combination with a relief valve.
By way of example, in a technical art disclosed in JP-A-2001-59428, a relief valve is provided in a relief passage that bypasses an exhaust throttle valve. In this technical art, the exhaust throttle valve is fully closed at the engine""s cold start for the purpose of reducing cold start emissions of unburned HC, and the relief valve opens when the exhaust pressure increases to reach a preset pressure of the relief valve, whereby the exhaust pressure is kept at the preset pressure.
Since the relief valve is usually kept closed, however, a dead-end alley is formed in the relief passage at the location of the relief valve. This allows moisture content of the exhaust gas to accumulate there, as condensed water, which contains various exhaust gas components such as Fe, Ni, S, etc. These components, as contaminants, adhere to sliding parts of the relief valve, etc., to increase sliding resistance thereof. In particular, sulfur components accumulated in an NOx catalyst have strong acidity to cause corrosion, thus largely affecting the sliding parts. This poses a problem that the relief valve is hindered from operating normally.
As a countermeasure for this, the exhaust throttle valve may be closed at regular intervals to increase the exhaust pressure, thus causing the relief valve to operate (open) so as to permit condensed water accumulated in the dead-end alley of the relief passage to be discharged therefrom, thereby removing contaminants adhered to the sliding parts, etc., together with the condensed water. However, the closure of the exhaust throttle valve can increase the engine""s exhaust resistance, posing another problem of deteriorated fuel consumption, and hence preventive measures therefor have been desired.
An object of the present invention is to provide an exhaust purifying apparatus for an internal combustion engine capable of operating a relief valve to discharge condensed water accumulated in a relief passage therefrom, thus avoiding faulty operation of the relief valve attributable to contaminants contained in the condensed water, and capable of suppressing deteriorated fuel consumption caused by increased exhaust resistance upon operation of the relief valve.
Another object of the present invention is to provide a failure prevention method for an exhaust purifying apparatus of an internal combustion engine that is capable of avoiding faulty operation of a relief valve without making fuel consumption deteriorated.
According to one aspect of this invention, there is provided an exhaust purifying apparatus of an internal combustion engine, which comprises: an exhaust throttle valve provided in an exhaust passage of the internal combustion engine, so as to be able to be closed to close the exhaust passage; a relief passage provided communicating with the exhaust passage and bypassing the exhaust throttle valve; a relief valve provided in the relief passage so as to be able to be closed to close the relief passage, and arranged to be opened to open the relief passage when receiving an increased pressure in the exhaust passage that is closed with closure of the exhaust throttle valve; fuel cut determining means for determining whether the internal combustion engine is during fuel cut-off; and a relief-valve forcibly operating means for closing the exhaust throttle valve to open the relief valve when it is determined by the fuel cut determining means that the internal combustion engine is during fuel cut-off.
According to another aspect of the present invention, there is provided an exhaust purifying apparatus of an internal combustion engine, which comprises: an exhaust throttle valve provided in an exhaust passage of the internal combustion engine, so as to be able to be closed to close the exhaust passage; a relief passage provided communicating with the exhaust passage and bypassing the exhaust throttle valve; a relief valve provided in the relief passage so as to be able to be closed to close the relief passage, and arranged to be opened to open the relief passage when receiving an increased pressure in the exhaust passage that is closed with closure of the exhaust throttle valve; deceleration determining means for determining whether the internal combustion engine is during deceleration; and a relief-valve forcibly operating means for closing the exhaust throttle valve to open the relief valve when it is determined by the deceleration determining means that the internal combustion engine is during deceleration.
According to still another aspect of the present invention, there is provided a failure prevention method for an exhaust purifying apparatus of an internal combustion engine, which comprises the steps of: (a) determining whether or not the internal combustion engine is during fuel cut-off or deceleration; and (b) closing an exhaust throttle valve so as to cause a relief valve to open when it is determined at the step (a) that the internal combustion engine is during fuel cut-off or deceleration, the exhaust throttle valve being provided in an exhaust passage of the internal combustion engine, for closing the exhaust passage to increase a pressure in the exhaust passage, and the relief valve being provided to be able to close a relief passage that is provided communicating with the exhaust passage and bypassing the exhaust throttle valve.
Moisture content of exhaust gas is accumulated, as condensed water, at a location of the relief valve in the relief passage, and condensed water components, as contaminants, are gradually adhered to the relief valve. On the other hand, the exhaust throttle valve is caused to be closed by the relief-valve forcibly operating means, and the resultant increased pressure in the exhaust passage causes the relief valve to open, so that the contaminants adhered to the relief valve are removed, together with the condensed water that is discharged from the relief passage. Thus, faulty operation of the relief valve can be avoided. Since the closing of the exhaust throttle valve is carried out when the internal combustion engine is during fuel cut-off or during deceleration, the closure of the exhaust throttle valve hardly results in degraded fuel consumption though it makes engine""s exhaust resistance increase.